Typing the following strings into Firefox's Address Bar (which the new browser calls the Location Bar) and pressing Enter brings up a wide variety of novel applets:
about: shows info on Firefox's version number, copyright, etc.;
about:config reveals the Configuration Console, a repository brimming over with scores of customizable settings;
about:cache displays a summary of both your memory and file cache, with a link to full file listings;
about:buildconfig lists the compiler options that were used to create your version of Firefox (and, since it's open source, anyone can compile a customized version);
about:plugins enumerates your installed add-ons, which can be quite numerous since Firefox is designed to be modular and extensible; and
about:credits is an "Easter egg" that includes the names of hundreds of developers and testers who worked on the product.

You can edit these files without having to find their specific location by using a free, third-party utility called ChromEdit.xpi. After you install this extension and restart Firefox, a new Edit User Files item shows up on the Tools menu.

Fix a memory leak in Firefox 1.0
Firefox is supposed to dynamically release memory from its RAM cache to other Windows applications as needed. Unfortunately, Firefox 1.0 seems to consume more memory than it should, which hurts performance, when set to the default of 51200 KB (51 MB).
To solve this, Firefox power users recommend limiting the memory cache using the Configuration Console. This frees up memory for other apps, speeding up everything to a greater or a lesser extent, depending on your machine and the applications you run. Here's how the trick works:
Step 1. Type about:config into Firefox's Address Bar and press Enter.
Step 2. Right-click any row, then click New, Integer. Type or paste the following preference name into the dialog box that appears (this is a hidden preference that doesn't exist in the Configuration Console until you create it):
browser.cache.memory.capacity
Step 3. Click OK, then enter the following integer number into the next dialog box, representing 16 MB of RAM for the cache:
16000
Step 4. Click OK to close the dialog box, then close all instances of Firefox and restart it.
For a lengthy discussion of this option, see Mozillazine's forum topic 172041 http://forums.mozillazine.org/viewtopic.php?t=172041

Move the disk cache to a faster or larger location
Many Windows users like to locate their disk cache files on a separate hard drive from Windows. This can improve the performance of Windows or whatever program is using the cache, or it can free up space on a primary partition. (The disk cache should not be confused with the RAM cache described above.)
To move Firefox's disk cache, use about:config to create a new preference. (This is another hidden preference that doesn't appear until you create it.)
Type browser.cache.disk.parent_directory into the dialog box that appears, then click OK. In the Value dialog box, enter c:\folder, providing the foldername in which you wish the file to be located.
This and other networking and cache settings are sparely documented by a Firefox developer called Ben C. on his NetPrefs page.
//www.mozilla.org/quality/networking/docs/netprefs.html

Force frames to be resizable on Web pages
If you visit Web sites that divide their pages into rectangular frames, but some of the frames aren't wide enough for you to see all of the words, you can tell Firefox to put borders around all frames so they're resizable.
To do this, use about:config to change layout.frames.force_resizability from "false" to "true."

Speeding up the maximizing of Firefox
By default, when you minimize Firefox, it gives back to Windows most of the RAM the browser was using. It sometimes can take several seconds for the browser window to become fully loaded when you restore the window to its original size. This is one of those "version 1.0" things, in my opinion, that will become more streamlined in future bump revs of the program.
Fortunately, there's a hidden setting that can accomplish the restoration of Firefox windows much more quickly:
Step 1. Use about:config to create a new, Boolean value. Type or paste the following string into the dialog box that appears:
config.trim_on_minimize
Step 2. Click OK to close the dialog box. Change the value from "true" to "false" and restart Firefox.

Speeding up Firefox on Windows XP
You can speed up the loading of Firefox on Windows XP by adding it to the programs that XP "prefetches."
To do this, right-click the Start Menu item for Firefox, or any icon you use to start Firefox, and open the Properties dialog box. Add a space plus /Prefetch:1 to the command line and click OK. The resulting line might look as follows:
"C:\Program Files\Mozilla Firefox\firefox.exe" /Prefetch:1
A detailed explanation of XP's prefetch feature is provided by TechRepublic.
http://techrepublic.com.com/5100-6270_11-5165773.html?tag=e064

Use a "Moox" build that's customized for your CPU
A developer who goes by the name of Moox has compiled the Firefox code into separate executables, optimized for the instruction sets of different CPUs. This provides the biggest performance boost that I've seen (and also consider to be reliable). The developer's PDF white paper on the topic shows reductions of as much as 30% in the time required for Firefox to perform various tasks.
At this writing, there are three separate builds: one for Pentium 4 and AMD Opteron (and other CPUs), one for Pentium 3 and AMD Athlon MP, and one for Pentium 2 and AMD Athlon. The builds are free, of course.
To download the builds, see Moox's Mozilla page.
//www.moox.ws/tech/mozilla/